1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a device-to-device signal and a device-to-cellular signal and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of the universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (UL-MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Device-to-device (D2D) communication may be realized, after an initialization (e.g., connection establishment and/or peer discovery) is performed (e.g., assisted by an eNB). Then, two UEs may communicate (e.g., transmitting and/or receiving packets) with each other directly according to the D2D communication, and the eNB does not need to forward the packets transmitted between the communication devices. According to the D2D communication, the UEs may communicate with each other via UL resources (e.g., UL subframes configured by the eNB). In general, the D2D communication may also be seen as a D2D service (i.e., proximity service (ProSe)). In addition, a D2D subframe, D2D transmission, D2D communication and D2D discovery can be termed as a sidelink subframe, sidelink transmission, sidelink communication and sidelink discovery, respectively.
A UE may transmit a D2D signal and a device-to-cellular (D2C) signal at the same time (e.g., in the same subframe), when the UE supports both D2D communication and D2C communication. However, the D2D communication and the D2C communication may be performed according to different timings. For example, the D2D communication may be performed according to a DL timing, while the D2C communication may be performed according to a UL timing. It should be noted that the DL timing is used for receiving DL signals transmitted by the eNB, and the UE performs alignment of the DL timing by itself. The UL timing is used for transmitting UL signals to the eNB, and the UE performs alignment of the UL timing according to an indication (e.g., timing advance command) transmitted by the eNB. In this situation, a subframe (i.e., D2D subframe) for the D2D communication may overlap with a subframe (i.e., D2C subframe) for the D2C communication. More specifically, the last few single-carrier frequency-division multiple access (SC-FDMA) symbols of the D2D signal may overlap with the first few SC-FDMA symbols of the D2C signal. In such a situation, the D2D signal and the D2C signal may not be transmitted and received correctly, and the D2D communication and the D2C communication may not be performed regularly. The problem may be even severe when the D2C signal includes a sounding reference signal (SRS).
In another example, a total power level (e.g., total transmit power level) used by a UE may be greater than a power level limit of the UE, when the UE transmits a D2D signal and a D2C signal at the same time (e.g., in the same subframe). Accordingly, the UE may transmit the signals incompletely, i.e., with insufficient power levels, and the receptions of the signals may be affected.
Thus, how to multiplex (e.g., transmit) the D2D signal and the D2C signal is an important problem to be solved.